The present invention relates to a method of delignification of cellulosic fiber material.
U.S. Pat. No. 4,220,498 describes a method of delignification of cellulosic fiber material which is fed in a main line in order to be treated with oxygen in a main reactor. The pulp is screened at at least one point in the main line and the screen reject obtained is fed in a branch line in order to be treated separately with oxygen and then returned to the main line.
However, as a further necessary measure the screen reject is refined in order to reduce the shives content, before performing the separate oxygen treatment. Furthermore, the screen reject thus delignified must be returned to the main line at a point downstream of the main reactor, and more particularly downstream of the screening apparatus in which the screen reject was previously separated from the main line. The screen reject which is separated from the screening apparatus has a high kappa number since the bundles of fibers therein, known as shives, have not been delignified to the same extent as the other part of the pulp which is more easily accessible. Even if refining reduces the shives content, and further reducing is performed in the separate oxygen reactor, the main stream of pulp is supplied with a screen reject or shives stream which has a kappa number essentially higher than the kappa number of the main stream at the point of mixing. The pulp thus will have an undesirably high content of shives, the pulp quality will be uneven and increased quantities of chlorine are required in the bleaching department resulting in increased emission of injurious chlorine compounds. The refining required according to said patent affects the fiber length and thus also impairs the strength properties of the pulp when the shives stream rejoins the main stream. Refining also requires high amounts of energy as well as relatively expensive equipment.